The term, or physiological property called, “rectal capacity” refers to the volume of the rectum. This physiological property is measured approximately—for example by using a high compliance balloon of suitable geometry—by looking at the amount of air or liquid used to fill the balloon up to the point where the pressure that is required to further inflate the balloon rises appreciably above a baseline (e.g. near intra-abdominal pressure). The term, or physiological property called, “rectal compliance” is a measure of the distention elasticity of the rectum. It is taken as the measured change in pressure inside the balloon divided by the change in balloon fill volume (air or water), typically after the balloon has filled the un-stretched (un-distended) organ.
Rectal compliance and rectal capacity have significant bearing on lower gastrointestinal (GI) function and dysfunction (e.g., diarrhea and constipation). Rectal compliance and rectal capacity measurements are currently done as separate tests from the more commonly employed anorectal manometry studies that are based on physiological pressure that is measured/sensed in the anal canal and rectal cavity. For example, reduction in the compliance of the rectum, which shortens the time between sensation of stool and the urgent need to have a bowel movement, may cause or result in fecal incontinence. Surgery or radiation injury can scar and stiffen the rectum. Inflammatory bowel disease can also make the rectum less compliant. Currently, anorectal manometry catheters that are designed for anal and rectal evaluation use a short balloon that does not enable evaluation of the rectal capacity and rectal compliance.